1. Field of the Invention
This invention relates to a novel process for curing a resin-impregnated cellulosic substrate by employing continuously superheated steam. This invention is particularly applicable to the curing of a phenolic resin impregnated paper to thereby afford a cured paper product having improved wet properties.
2. Description of the Prior Art
The impregnation of substrates with synthetic resins to alter and, hopefully, improve certain properties of said substrate is well known in the art. For instance, in the paper industry it is known to treat the fibers of a cellulosic material with various synthetic resins, i.e., urea-formaldehyde resin, melamine resin, phenol-formaldehyde resin, and the like, to produce cellulose products having improved wet properties (e.g., boil strength). Once the cellulose material has been impregnated with the resin, the impregnated substrate is generally subjected to a treatment to effect curing of said resin.
Many different methods of curing resin impregnated cellulose substrates are known and recognized in the art. For example, see U.S. Pat. No. 2,639,242, issued to Suen, which discloses the use of radiant heat to cure the resin. Compare U.S. Pat. No. 2,785,975, issued to Sheeran, which discloses heating a resin impregnated cellulosic substrate for twenty-four hours at 250.degree. F. in order to cure the resin. The extended residence time of twenty-four hours is a large disadvantage of the process as the process tends to be too time consuming. More conventional methods have thus employed a higher temperature of about 300.degree. F. in order to shorten the residence time to about 15 minutes in a hot air oven. A 15-minute cure time, however, is also somewhat time consuming and a still shorter residence time would be desirable.
U.S. Pat. No. 3,761,977, issued to Rappoport, discloses the use of steam for treating and curing textiles, but not resin impregnated cellulosic substrates. The steam is employed at high pressures, i.e., about 5 psig., and is passed through the textile substrate. The passage of steam directly through a cellulosic substrate, however, is not always plausible or possible as the substrate may be harmed, particularly at high pressures. The use of a high pressure is also disadvantageous as it requires the use of special pressure equipment.
U.S. Pat. No. 2,565,152, issued to Wachter et al, discloses the use of saturated steam to not actually cure the resin, but provide increased penetration of the phenolic resin into the fibrous material upon condensation of the steam onto the substrate. Subsequent to the steam treatment, the cellulose substrate is heated in an oven to remove the water and also cure the resin. The patent discloses that the steam treatment reduces the gelation time of the resin.
In addition to the aforementioned problems and disadvantages, the prior art processes have also failed to address the problems of recovering the vaporous by-products of the curing process, e.g., phenol vapors, in a commercially acceptable manner. Generally, these vapors are merely dispersed and lost to the atmosphere, which can have a detrimental ecological impact.
Furthermore, although conventionally cured resin impregnated cellulose substrates do show somewhat improved wet properties, the hard-cured paper obtained is generally brittle and of inferior flex properties.
Accordingly, it is a major object of this invention to provide a novel process for curing a resin impregnated cellulosic substrate which is devoid of the aforenoted disadvantages of the prior art.
Another object of this invention is to provide a process for curing a resin impregnated cellulosic substrate which will afford the cured product improved physical properties.
Another object of this invention is to provide a process for curing a resin impregnated cellulosic substrate wherein all of the vaporous by-products of the curing process can be completely and easily recovered.
Another object of this invention is to provide a process for curing a resin impregnated cellulosic substrate which requires only a short residence time.
Still another object of this invention is to provide a curing process for a resin impregnated cellulosic substrate which can be run effectively under low pressures, i.e., less than 5 psig.
Other objects, features and the advantages of this invention will become apparent to those skilled in the art upon a study of this disclosure, the appended claims, and the drawing.